# current 当前的
current_users = ['A', 'b', 'c', 'd', 'e']
new_users = ['f', 'g', 'h', 'i', 'a']
lower_current_users = [current_user.lower() for current_user in current_users]
for new_user in new_users:
    if new_user.lower() in lower_current_users:
        print('Please enter other usernames.')
    else:
        print('The username has not been used.')